Conventionally rubber, both natural and synthetic, has been cured or vulcanized by heat treatment in the presence of sulfur or sulfur containing compounds such as mercaptans. By these means, which are well known, very good physical properties including tensile strength, modulus, elongation at the break, and the like, have been achieved. The techniques have been used commercially with a wide variety of synthetic rubbers prepared by both emulsion and solution polymerization.
Surfur and most sulfur containing compounds, unfortunately, have characteristically offensive odors. The art has long sought to avoid this problem, and has succeeded to some extent by the use of sulfur containing compounds of rather complex organic structure. There has been no decrease in the desirable properties of vulcanized rubbers as a result of the use of these compounds. However, other problems remain. Pressure molds are required for heat processing. The energy requirements are high. Heat processing of this type is difficult to automate.
For these and other reasons, the radiation curing or vulcanization of rubber has attracted considerable attention. With radiation, there is no odor problem and the procedure, as applied to other substrates, has been shown to be fast, clean and efficient . . . well adapted to modern, high speed, mass production, commercial operations. The process is readily automated, the energy requirements are appreciably reduced, and pressure molds are not required. However, despite a great deal of work and many promising literature reports, radiation procedures have not been successfully adapted to the commercial vulcanization of rubber. It has proved to be difficult, at any economically feasible radiation dose, to produce vulcanized rubbers having physical properties as good as rubbers vulcanized with ordinary sulfur heat cures.
The principal problem which has plagued investigators attempting to adapt radiation techniques to the curing of rubber is that it has not proved possible to effect radiation cure of substantially pure synthetic rubbers at a low average dose, for example up to 7 megarads. The value of 7 megarads for the average dose, i.e., the average dose throughout the thickness of the substrate, is important for the reasons stated below. For a proper understanding of this invention, average dose and surface dose should be carefully distinguished. The former refers to the average radiation dose throughout the thickness of the substrate. The latter refers to the dose at the surface. Normally an average dose of 9 megarads corresponds to a surface dose of about 7 megarads with relatively thin sheets of about 70 to 90 mils and an electron energy of approximately 1.5 Mev. (1) Many of the additives which are ordinarily added to rubber compositions in the course of compounding them for their ultimate commercial use are radiation cure inhibitors. As a result of the presence of these inhibitors, the average radiation dose required for curing is so high that the cost is prohibitive. If the pure synthetic rubber is to be used in the final composition cures at a dose level below 7 megarads, the presence of those additives which are normally employed in commercial rubber compounding will not inhibit curing by high energy ionizing radiation to the point where radiation curing is impractical.
(2) In some commercial operations, it is desirable to partially cure a rubber composition in one phase of the operation and to complete the cure in another. The partial curing may be effected by radiation. The cure may be completed by heat. The compound, therefore, will contain normal heat cure additives. The heat generated by treatment of synthetic rubber compositions increases with increasing radiation dose. If the dose to obtain a partial radiation cure is too high, the heat generated to reach this dose level may initiate the heat cure system to effect a partial heat cure as well. As a result, it would not be possible to properly control the partial cure.
(3) Radiation curing at low doses can be carried out without unnecessary increases in temperature. This is very important. Normally an increase of one Mrad in radiation dose corresponds to an increase in temperature of about 5.degree. C. Therefore if a relatively volatile material is present in the rubber to be cured, it is essential to maintain the radiation dose at a low level to prevent the development of undesirable porosity in the cured rubber as a result of "blowing" of the volatile material. For example, if blowing takes place at 100.degree. C. and radiation curing is initiated at 25.degree. C., the upper level of permitted dose is 15 Mrad.
(4) Side reactions such as polyene formation, scission, isomerization and cyclization which change the nature of the compound being treated are reduced to a tolerable level by avoiding high radiation dosages.
Curing of rubbers is effected by crosslinking the polymer chains. With sulfur cures crosslinking takes place through sulfur links. With radiation curing crosslinking is normally through carbon-carbon bonds of adjacent polymer chains. As a result of such crosslinking, the rubber becomes less tacky and physical properties such as modulus, tensile strength, elongation at the break, ultimate elongation and other desirable properties improve. No radiation curing procedure has yet been devised which, without special formulations which increase cost and complexity, is capable of curing synthetic rubbers to produce finished products having physical properties comparable with sulfur cured products.